This disclosure is directed to a method and apparatus for positioning optical fibers in underground conduits. Optical fiber transmission is adventagous in telephony. It provides clarity of signal transmission and also an extremely wide band width to enable a tremendous reduction in size of telephone cables. Thus, a relatively small optical fiber can replace a large armored telephone conduit of the sort typically supported on telephone poles or alternately, buried underground in specially constructed tile culverts for the conduit. Typically, the conduit encases several hundred, perhaps several thousand pair of conductors, each pair being connected to an individual telephone customer. As will be understood, the conductor pairs do not carry substantial current but they do have to provide a relatively noise free connection for the various customers. Installation and subsequent maintenance over a period of years costs substantial sums for armored conduit cabling. Optical fibers have the advantage of reduced size and greater band pass and hence are replacing the armored conduit cable.
In the past, a typical telephone cable has been buried underground exercizing caution to position the conduit in a tile culvert in some installations, or to otherwise provide a suitable wrapping to prevent such ground water intrusion. Ground water intrusion creates substantial noise. In many instances, it can be kept out of the cable only by continuously maintaining a positive pressure of dry nitrogen in the armored cable. Many advantages thus arise from installing optical fibers for telephony transmission for hundreds, perhaps even thousands of conductor pairs.
Optical fibers can be fabricated substantially without limit to length. However, at the time of installation, they must be carefully spliced, and splicing of an optical fiber typically is a tedious and rather expensive undertaking. Accordingly, it would be adavantageous to install optical fibers in great lengths with fewer splices to reduce the cost of installation. Presently, optical fibers are installed and routed underground by means of a plastic pipe. The plastic pipe has a wall specifically made to prevent accidental puncture and is substantially impervious to the intrusion of underground water. In fact, optical fibers are impervious to underground water. However, there is a problem that arises from the installation procedure. Assume (as an example to set forth one problem) that an underground plastic pipe has a length of 10,000 ft. The present approach utilizes a shuttle device to pull a leader into the plastic pipe. The shuttle device is forced by air into the plastic pipe and will travel some distance. Distances of 1,000 ft. are not uncommon. In fact, the shuttle of the prior art (to be discussed below with regard to FIG. 1) has occasionally achieved more distance and has been reported to sometimes accomplish 1800 ft. This then requires a splice to be made. Leader length in the pipe can be measured quite easily and hence the end of the leader can be determined. This then requires that the plastic pipe be dug up to locate the towing device (known as a birdie). At this point, the splice is made and the birdie then continues further down the pipe. One birdie trip may not fully span the length of the plastic pipe which in this example is 10,000 ft. Assuming that the birdie travels 1500 ft., it may well require interruption of the plastic pipe (with splicing) to fully extend the optical fiber along the 10,000 ft. of plastic pipe. Several such interruptions will be required. If an average travel was 1500 ft., 6 intermediate digging operations would be required to retrieve the birdie and perform the necessary steps prior to launching the birdie. Again each digging operator would be accompanied by a splice.
The apparatus and method of the present disclosure shows a marked improvement in range over the previous birdie system. This improvement enables the optical fiber to be extended over greater distances without interruptions, and hence with a minimum of pipe digging and splicing. Thus splices are avoided, reducing the cost of installation. This apparatus thus enables the leader to be towed through a greater length of plastic pipe when installing the optical fiber system.
Certain advantages in addition to these will be noted on a description of the preferred embodiment and method of using the present apparatus. The present apparatus is thus summarized as an optical fiber leader towing pig having multiple walls and permiting controllable blow-by. The blow-by is quite large when the pig is first introduced into the plastic pipe. As the pig travels through the plastic pipe, the leader which it tows creates drag and provides a retarding force on the pig. The pig is constructed and arranged so that the towing drag or force acting on the pig causes the pig to swell at the forward end and to have an enlarged diameter. As the diameter at the forward end of the pig enlarges, the amount of blow-by is reduced, thereby increasing the propelling force acting on the optical fiber towing pig. This increases the pulling force of the pig thereby increasing the range of travel. An increase in range is reflected as an increase in length of optical fiber leader which can be pulled through the plastic pipe.